Electric motors are electrically noisy, the sparks commonly seen at the brushes are a source of radio frequency interference [RFI] that can interfere with both the host controller and other
systems. Good motor noise suppression will improve system reliability by reducing the chance of MOSFET latch up and failure.Have a look at this video…..
If you want to see some arcs in super slow motion, here is a great video.
Some steps to take to prevent RFI are;
- Make sure each of your motors has a suppression capacitor fitted. A small ceramic capacitor 10nF / 100v is often fitted internally across the motor brushes. If your motor does not already have one, fit one externally across the motor connections as near to the motor as possible. There’s more information on this subject in the page Radio Controlled Machines: General wiring hints
- Fit ferrite rings to the motor leads.
- Twist the motor leads together if possible [this stops them acting like a loop aerial].
- Keep the motor leads away from other wiring as far as possible. They are carrying significant currents that are being switched very quickly. This creates electrical noise which can be coupled into any adjacent wiring.
- If your motor is subject to shock loads or fast acceleration / deceleration [such as in Robot Wars] consider fitting a fast acting varistor transient suppressor across the motor terminals.
- If your system is part of an automotive installation that includes relays, fans, or other motors that can produce noise spikes, then consider fitting a transient suppressor across the B+ / B- terminals. This Littlefuse page give more information on these.
- If you have inductive horns fitted to your vehicle, be aware that these can also cause electrical noise. We recommend you fit a “catching” diode across each horn. Diode type 1N4002 or similar is readily available, and should be fitted with the white band connected to the horn +ve.
- Keep the motors clean, dust and metallic particles around the brushes will increase wear, sparking, and electrical noise.
- Make sure that the brushes and armature do not become too worn. Brushes that are worn will have a light spring pressure which will lead to more sparking which is electrically noisy.
- See this article on good wiring practise.
Noise suppression fitted to one of our test rigs. This is in daily use and we’ve not had a single mosfet fail on it yet.
We’ve put together some interference suppression packs that have the components we recommend.
For extreme cases, additional RFI hardening techniques include painting the inside of the case with nickel paint such as this from Farnell , and fitting screened control cables.
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